Evidence from our laboratory indicates that arginine vasoressin (AVP) may participate in the regulation of arterial pressure and that many subjects with essential hypertension exhibit elevated levels of plasma AVP. There are tow specific aims outlined in the present grant proposal. First, to validate and better characterize plasma AVP levels in essential hypertension and to study sufficient numbers of subjects to determine the relationships between plasma AVP, urine concentratinb ability, sodium intake, plasma renin activity, circulating catecholamines, and the influence of age, sex and race on these variables. Secondly, to examine four hypotheses regarding the reasons for AVP elevations in hypertensive man: 1) To determine if plasma AVP is elevated due to an increased osmotic sensitivity or lowered threshold inhypertensive man, the relationship between plasma osmolality and plasma AVP will be determined using osmotic forcing functions. 2) To determine whether hypertensive subjects respond to nonosmotic stimuli with a greater release of AVP than normotensive subjects, tilt-board studies will be carried out for comparison of changes in plasma AVP and catecholamines. 3) To determine if AVP may be secondarily elevated to compensate for reduced renal concentrating ability, experiments are designed to determine if vascular reactivity to AVP is increased in hypertensive subjects to a greater extent than reactivity to other vasoconstrictors such as norepinephrine. Changes of forearm vascular resistance with graded infusions of AVO will be compared to resistance changes observed with norepinephrine in normotensive and hypertensive subjects. These studies represent one of the first major efforts to characterize the behavior of this major endocrine control system in hypertensive man.